In vitro and in vivo hMSCs can differentiate into osteoblasts, adipocytes, and chondrocytes (Friedenstein et al., 1976, Exp Hematol 4:267-274; Friedenstein et al., 1987, Cells Tiss Kin 20:263-272; Pereira et al., 1995, Proc Natl Acad Sci 92:4857-4861; Pittenger et al., 1999, Science 284:143-147; Sekiya et al., 2002, Proc Natl Acad Sci USA 99:4397-4402; Sekiya et al., 2004, J Bone Miner Res 19:256-264) but they also adopt a stromal role, by providing extra cellular matrix components, cytokines, and growth factors for paracrine tissue support (Dexter et al., 1984 Blood Cells 10:315-39; Austin et al., 1997 Blood 89:3624-3635; VanDenBerg et al. 1998, Blood 92:3189-3202). As the presumptive precursors of osteoblasts, hMSCs and related cell lines provide a convenient cell culture model for the study of osteogenic tissue repair in an experimentally accessible system (Gregory et al., 2004, Anal Biochem 329:77-84). Early studies, employing cultures of osteogenic progenitors have yielded a wealth of information describing the molecular events that modulate osteogenic differentiation. A critical finding of these studies is that positive signaling by the canonical wingless (Wnt) pathway is essential for differentiation into osteoblasts (Bain et al., 2003, Biochem Biophys Res Commun 301:84-91; Rawadi et al., 2003, J. Bone Miner Res 18:1842-1853; Gregory et al., 2006, Drug News Pers 19:445-452). In the canonical Wnt signaling pathway, secreted Wnt ligands bind to the receptor frizzled (Frz) and the co-receptor lipoprotein-related protein 5 and 6 (LRP-5/6) on the target cell. Activation of Frz recruits the cytoplasmic bridging molecule, disheveled (Dsh), so as to inhibit the action of glycogen-synthetase-kinase-3β (GSK3β). Inhibition of GSK3β decreases phosphorylation of β-catenin, preventing its degradation by the proteosome. Stabilized β-catenin acts on the nucleus by activating T-cell factor/lymphoid enhancing factor mediated transcription of target genes that elicit a variety of effects including induction of differentiation and in some cases, proliferation. Canonical Wnt signaling is tightly regulated by a combination of positive induction through the binding of the Wnt ligand and negative regulation through numerous mechanisms including the secreted glycoprotein dickkopf-1 (Dkk-1) (Tian et al., 2003, N. Engl. J. Med. 349:2483-2494).
The clinical significance of Wnt signaling in osteogenesis has been highlighted by reports that mutations in LRP5 that prevent Dkk-1 binding, cause abnormally high bone density (Boyden et al., 2002, N Eng J Med 346:1513-1521) and mutations that render LRP5 functionally null cause a form of osteoporosis (Gong et al., 2001, Cell 107:513-523).
MSCs from human bone marrow have the capacity to regenerate bone when administered at the site of injury. However, their efficacy is limited because the cultures of MSCs employed are frequently functionally heterogeneous. Therefore a proportion of the administered cells do not contribute to repair of the bone tissue.
In many cases, serious bone trauma does not heal because the damage is too severe to permit the normal process of repair. Serious non-healing bone injuries are usually treated by one or a combination of the following techniques; prosthetic implantation, e.g. plates, pins, or screws, bone graft using autologus explantation, then implantation at the site of injury, or bone morphogenic protein (BMP) implantation. Prosthetic implants are generally straightforward and can be effective, but the body cannot maintain the material as it does with bone tissue. Therefore, weight bearing prosthetics are susceptible to wear. Furthermore, in some cases bone degradation occurs at the site and the implants can be rejected. Bone autografts have the obvious drawback of requiring donor material, limiting the size of the implant. BMPs can accelerate the inherent healing capacity of bone, but if the bone is destroyed, BMPs cannot repair the injury.
There is a need in the art for novel therapeutic formulations of MSCs. Specifically, the prior art is deficient in compositions of MSCs effective to stimulate osteogenesis at a site of bone degeneration or bone injury. The present invention fulfills this long-standing need and desire in the art.